Japanese Unexamined Patent Application Publication No. Hei 08-505218 “Method and Apparatus for Measuring Concentration of Carbon Dioxide Dissolved in Seawater” (Patent Document 1) discloses a technique in which a peak absorption wavelength λM, a wavelength λB at which dye does not absorb light, and a wavelength λI close to the isosbestic point of the dye are measured from a light absorption spectrum, the carbon dioxide concentration is measured at only one wavelength λM with absorption peak and corrected at λB at which the dye does not absorb light, and the stability of the indicator is evaluated on the basis of measurement at the wavelength λI close to the isosbestic point of the dye.
Japanese Unexamined Patent Application Publication No. 2002-514758 “System and Method for Optical Chemical Detection” (Patent Document 2) discloses a technique in which the intensities of first and second output light are measured to determine the concentration of an analysis subject (carbon dioxide: CO2).
A paper titled “Simultaneous Vertical Measurements of In Situ pH and CO2 in the Sea Using Spectrophotometric Profilers” (Non-Patent Document 1) presented by the present inventor in the Journal of Oceanography, Vol. 62, pp. 71 to 81, 2006 proposes a method for measuring the concentration of carbon dioxide in seawater as described below. A pH indicator solution changes in color in accordance with changes in pH, the pH indicator solution having a carbon dioxide concentration that finally becomes equal to the concentration of carbon dioxide in water, and the pH indicator solution establishes an equilibrium of:HL−⇄H++L2−  [Chemical formula 1]if the pH indicator solution is represented as H2L.
First, an absorption spectrum of light absorbed by the pH indicator solution is measured. Next, a first absorbance A1 at a first peak absorption wavelength λ1 which is equivalent to a concentration of HL− in the pH indicator solution, a second absorbance A2 at a second peak absorption wavelength λ2 which is equivalent to a concentration of L2− which determines a pH, an isosbestic point absorbance A0 at an isosbestic point wavelength at which the pH indicator solution does not show changes in absorbance in accordance with changes in pH, and a non-light-absorbing absorbance Ab at a non-light-absorbing wavelength λb are computed from the absorption spectrum. Then, a pH value of the pH indicator solution is computed using a basic pH computation expression for computing the pH value of the pH indicator solution on the basis of a ratio (A1−Ab)/(A2−Ab) of a difference (A1−Ab) between the first absorbance A1 and the non-light-absorbing absorbance Ab to a difference (A2−Ab) between the second absorbance A2 and the non-light-absorbing absorbance Ab. Finally, the concentration of carbon dioxide in water is determined from the computed pH value.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. Hei 08-505218    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-514758    [Non-Patent Document 1] Journal of Oceanography, Vol. 62, pp. 71 to 81, 2006, “Simultaneous Vertical Measurements of In Situ kpH and CO2 in the Sea Using Spectrophotometric Profilers”